The whole reason UA and other airlines (including LCCs) fly narrowbodies, especially smaller narrowbodies like 737s, is that customers demanded more frequency. That's one reason for the proliferation of RJs - high frequency, hence lower capacity per plane.
UA is trying to cut capacity, and part of that involves cutting frequency... but cutting it by combining 3-4 narrowbodies into a single widebody is probably just a bit more than they are trying for (as well as not having enough widebody planes to do it).